In the field of therapeutic fluid delivery, various approaches to instrumentation for controlling the infusion rate have been proposed. Some of the instrumentation previously disclosed utilizes a metering chamber divided into two compartments by a movable membrane. For example, U.S. patent application Ser. No. 404,811, filed Aug. 3, 1982, and owned by the owner of the present application, describes in detail a number of such systems for achieving volumetric accuracy of delivery. That application discloses a disposable cassette containing a dual chamber, with each compartment of the chamber being provided with a valved inlet and outlet. Upon opening of a valve pair comprising an inlet and an outlet on opposite sides of the membrane separating the two compartments, exactly one chamber's volume of fluid is delivered to the patient, and then fluid flow stops. The alternation of opening and closing of the two pairs of valves thus provides a digitized flow of fluid in precise volumetric quantities. By selecting the rate at which the valve states are initiated, the user may provide for the precise desired overall volumetric flow rate.
Another patent of the owner of this invention, U.S. Pat. No. 4,431,425, discloses a membrane sensing apparatus for detecting flow faults in such a device by sensing changes in position of the chamber membrane. The invention of this application is directed to a method for utilizing such non-invasive membrane position detection to provide monitoring information on the status of flow.
Patent art of interest in the general field of fluid flow control utilizing chambers having a movable membrane may be found in U.S. Pat. Nos. 4,204,538, 4,207,871, 4,121,584, and in U.S. patent application 2,043,594 (published Oct. 8, 1980). A system for monitoring flow status in other types of infusion devices is disclosed in U.S. Pat. No. 4,307,598.